drm/amd/display: Mark function 'optc3_wait_drr_doublebuffer_pending_clear' as static

Compiling AMD GPU drivers displays a warning:

drivers/gpu/drm/amd/amdgpu/../display/dc/dcn30/dcn30_optc.c:294:6: warning: no previous prototype for ‘optc3_wait_drr_doublebuffer_pending_clear’ [-Wmissing-prototypes]

Get rid of it by marking the function as static

Signed-off-by: Caio Novais <caionovais@usp.br>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>